Compact scanning electron microscope

ABSTRACT

A compact electron microscope uses a removable sample holder having walls that form a part of the vacuum region in which the sample resides. By using the removable sample holder to contain the vacuum, the volume of air requiring evacuation before imaging is greatly reduced and the microscope can be evacuated rapidly. In a preferred embodiment, a sliding vacuum seal allows the sample holder to be positioned under the electron column, and the sample holder is first passed under a vacuum buffer to remove air in the sample holder.

The present application is a continuation of U.S. patent applicationSer. No. 12/303,611, with a 371 filing date of Jan. 22, 2009, whichclaims priority from International Application PCT/US2007/070655, filedJun. 7, 2007, and from U.S. Provisional Application 60/811,621, filedJun. 7, 2006, which are hereby incorporated by reference.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to electron microscopes.

BACKGROUND OF THE INVENTION

Electron microscopy provides significant advantages over opticalmicroscopy, such as higher resolution and greater depth of focus. In ascanning electron microscope (SEM) a primary electron beam is focused toa fine spot that scans the surface to be observed. Secondary electronsare emitted from the surface as it is impacted by the primary beam andsome electrons from the primary beam are backscattered. The secondary orbackscattered electrons are detected and an image is formed, with thebrightness at each point of the image being determined by the number ofelectrons detected when the beam impacts a corresponding spot on thesurface.

Electron microscopes are typically large, complex, and expensiveinstruments that require skilled technicians to operate them. SEMdevices typically cost well over $100,000 and require specialfacilities, including dedicated electrical wiring and venting of thevacuum pump outside of the operator area. Further, it can be difficultin a high magnification image such as that of an SEM for a user todetermine where on the sample an image is being obtained and tounderstand the relationship between that image and the rest of thesample. The cost of electron microscopes and the sophistication requiredto operate them have limited their use to research and industry that canafford the cost and provide the expertise to operate.

Because air molecules interfere with a beam of electrons, the sample inan electron microscope is maintained in a vacuum. After a sample isinserted, it typically takes a relatively long period of time for air inthe chamber to be evacuated, so that a user must wait before an image isavailable. This delay makes use of an SEM impractical in manyapplications.

It would be desirable to provide a low cost electron microscope that canbe operated by users that are not highly skilled and that could producean image quickly after a sample is inserted.

SUMMARY OF THE INVENTION

An object of the invention is to provide a scanning electron microscopethat is inexpensive, easy to use, and can be of sufficiently smalldimensions that it can be placed, for example, atop a classroom table.

An instrument of the present invention microscope uses a removablesample holder having walls that form a part of the vacuum region inwhich the sample resides. By eliminating the large sample chamber usedin prior art instruments, the instrument can be rapidly evacuated andimaging or other processing can be commenced rapidly.

The foregoing has outlined rather broadly the features and technicaladvantages of the present invention in order that the detaileddescription of the invention that follows may be better understood.Additional features and advantages of the invention will be describedhereinafter. It should be appreciated by those skilled in the art thatthe conception and specific embodiment disclosed may be readily utilizedas a basis for modifying or designing other structures for carrying outthe same purposes of the present invention. It should also be realizedby those skilled in the art that such equivalent constructions do notdepart from the spirit and scope of the invention as set forth in theappended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the exterior of a preferred embodiment of an electronmicroscope system of the present invention.

FIG. 2 shows a partial cross-sectional view of a portion of the electronmicroscope system of FIG. 1.

FIG. 3 shows another cross-sectional view of the electron microscopesystem of FIG.

FIG. 4 shows a first sample container used with the electron microscopesystem of FIG. 1.

FIG. 5 shows a second sample container used with the electron microscopesystem of FIG. 1.

FIG. 6A shows schematically the layout of the lower portion of theelectron microscope system of FIG. 1 with the sample holder positionedunder an optical navigation camera.

FIG. 6B shows schematically the layout of the lower portion of theelectron microscope system of FIG. 6A with the sample holder positionedunder a pre-evacuated vacuum chamber.

FIG. 6C shows schematically the layout of the lower portion of theelectron microscope system of FIG. 6A with the sample holder positionedunder a scanning electron microscope.

FIG. 7 shows an embodiment of a sliding vacuum seal used in the electronmicroscope system of FIG. 1.

FIG. 8 shows a schematic diagram of the sliding vacuum seal of theelectron microscope system of FIG. 1.

FIG. 9 shows a screenshot of the graphical portion of the user interfaceaccording to a preferred embodiment of the present invention, depictingthree image windows and various selectable buttons of the main screen;

FIG. 10 shows the screenshot of FIG. 9 with an image taken by theoptical navigational camera shown in the smaller optical overview windowand a magnification of the image shown in the larger main viewing windowwith a perimeter indicator imposed on the images;

FIG. 11 shows the screenshot of FIG. 10 with the perimeter indicatormoved to a different area of the image;

FIG. 12 shows a screenshot with a the same low magnification electronmicroscope image of the area selected on the optical window of FIG. 11shown in the large main image window and in the smaller electron beamoverview window;

FIG. 13 shows the screen shot of FIG. 12 with the main image screenshowing a higher magnification electron microscope image area of thearea indicated by the perimeter indicator in the electron beam overviewwindow;

FIG. 14 shows a screenshot of the archive screen of the user interfaceaccording to a preferred embodiment of the present invention; and

FIG. 15 shows a screenshot of the settings screen of the user interfaceaccording to a preferred embodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 shows the exterior of a preferred embodiment of an electronmicroscope system 100 of the present invention that includes SEMassembly 102, a display monitor 104, preferably incorporating a touchscreen 106 for accepting user input, an rotary user input device 108,and an external pre-vacuum pump 110, such as a diaphragm pump. A sampleholder 112 holds the sample to be viewed and is inserted into SEMassembly 102.

Electron microscope system 100 require no special facilitiesinstallation, that is, a preferred embodiment can operate onconventional power, that is, by plugging into a wall socket, and thevacuum system does not require venting to outside of the operator area.SEM assembly 102 does not require special vibration damping mounting.SEM assembly 102 is literally a “table-top SEM,” that can be set on anysturdy work surface and plugged into the wall. Electron microscopesystem 100 is therefore suitable for use in classroom or even in homes.Some embodiments can operate on direct current, such as from a 24 Vpower supply, making those embodiments truly portable.

FIGS. 2 and 3 show several of the subsystems of SEM assembly 102. Suchsubsystems include an electron source assembly 202, a condenser lenssection 204 and an objective lens section 206.

Electron Source Assembly

FIG. 2 shows electron source assembly 202, which includes an electrongun 212 comprising a thermionic emission source 214, such as a lanthanumhexaboride or cerium hexaboride crystal, partly surrounded by a Wehneltcap 216. Both the thermionic emission source 214 and the Wehnelt cap 216are maintained at a relatively high negative voltage, typically around−5,000 V, with the Wehnelt cap 216 biased a few hundred volts negativewith respect to the source. The Wehnelt 206 cap condenses the electronbeam and passes it through its aperture 218 toward an anode 220 that istypically maintained at ground potential.

Alignment Rods

Electrostatic alignment rods 222 align the beam from the source assembly202 with the optical axis of the lens system to compensate for anymechanical misalignment between the source 214, Wehnelt aperture 218,and anode 220. The alignment rods 222 can tilt the beam axis to provideprecise alignment, thereby reducing the required tolerance on themechanical alignment of the system components and reducing manufacturingcosts. Magnetic alignment rods or plates could also be used. In someembodiments, two sets of rods are used to allow the beam to be bothshifted and tilted. Using a single set of rods can reduce the cost andcomplexity of some embodiments.

Condenser Lens Section

SEM assembly 102 includes a condenser lens 230 that uses a magneticfield from a permanent magnet 232 to condense the electron beam. Asecond permanent magnet 234 provides a magnetic field for the objectivelens. A magnetic circuit constrains the magnetic flux from the permanentmagnets so that it can be used in both the objective lens and thecondenser lens. Condenser lens pole pieces provide the magnetic flux inthe electron beam path to condense the electron beam.

Objective Lens Section

SEM assembly 102 uses a permanent magnetic objective lens 240 instead ofan electromagnetic lens as used in most SEMs. As describe above, thepermanent magnets 232 and 234 provide the magnetic flux to objectivelens pole pieces 242. A focusing coil 244 provides additional magneticflux through the pole pieces 242 to alter the magnetic field to whichthe electron beam is subjected. The focusing coil 244 is used to changethe focal plane of the system, for example, when changing to a differenttype of sample holder 112, and to provide for fine adjustment to bringthe sample into focus. In a preferred embodiment, the permanent magnets232 and 234 provide sufficient flux to focus the beam at the system'sshortest focal length. The magnetic field from the focusing coil isoriented opposite to the field from the permanent magnets, therebyreducing the flux from the pole pieces and making the beam focus at agreater distance.

Electric Feedthrough

In any electron microscope, it is necessary to apply voltages within thevacuum system, and several means have been employed in the prior art totransfer power and signals from outside the vacuum chamber to inside thevacuum chamber. SEM assembly 102 uses a circuit board 250 sandwichedbetween the condenser lens section 204 and the objective lens section206 to provide power and electrical signals from outside the vacuumchamber to elements within the vacuum chamber. The circuit board 250 maybe a rigid or flexible board. The electric feed though is described inPCT/US2006/041976 for “Hermetically Sealed Housing with ElectricalFeed-In,” which is hereby incorporated by reference.

Deflector/Stigmator Rods

Electron microscope system 100 uses a single stage deflector 260positioned before the objective lens 240. By positioning the deflector260 before the objective lens 240, the working distance, that is, thedistance between the final lens and the sample is reduced, therebyimproving resolution. The beam is deflected using deflector rods 262that are attached to feed-through circuit board 250. Eight deflectorrods 262 comprise an electrostatic octupole deflector. The deflectorrods 262 are preferably 3 mm to 4 mm in diameter and about 2 cm to 3 cmin length. The rods are soldered to edge connectors on the printedcircuit board. Electron microscope system 100 preferably uses analograther than digital deflection electronics that are located on a secondcircuit board (not shown) that plugs into the feed-through circuit board250. Feed-through circuit board 250 uses gold plating as a ground planeto provide electrical isolation. The deflection electronics transfer thedeflection signal to the circuit board by a connector. By using eightrods, that is, an octupole design, the deflector 260 also functions as astigmator to correct astigmatism of the beam. Adjustment of the focusand stigmator may be performed manually or can be automated.

Detector

A preferred system uses a backscattered electron detector 280, which isan annular diode detector that is coaxial with the primary electron beamand which includes a hole through which the primary electron beampasses. A preferred detector 280 is divided into quadrants, and a usercan turn on or off individual quadrants or specify combinations, such asadditions or subtractions, of the signals from the different quadrants.Such manipulations can alter the image contrast to make the imaging moresensitive to difference aspects of the sample, such as topography orcomposition. Diode backscatter detectors are more suitable for operatingin the higher pressures of SEM assembly 102 than a typicalscintillator-photomultiplier secondary electron detector. Because of therelatively high pressure at the sample, some embodiments could us a gasamplification detector, such as those used in environmental scanningelectron microscope.

Sample Load/Unload System

A preferred embodiment of the present invention does not include aconventional sample vacuum chamber as was used in the prior art. In mostprior art systems, a sample stage is positioned in a sample vacuumchamber below the objective lens. The sample is inserted into the vacuumchamber, either through an air lock or by venting the chamber. Thesample chamber is then evacuated to an acceptable level of vacuum forimaging. When imaging is complete, the sample is removed from the samplestage and taken out of the vacuum chamber. The sample stage remains inthe system vacuum chamber.

Electron microscope system 100 does not include a conventional samplechamber at all. As discussed in greater detail below, the walls of theremovable sample container form part of the walls of a vacuum regioncontaining the sample.

FIGS. 4 and 5 show sample holders or sample containers 402 and 502,respectively, which are embodiments of sample holder 112 of FIG. 1.Sample holder 402 includes a conventional metallurgical mount 404 forholding a relatively flat sample, whereas sample holder 502 includes aconventional stub mount 504 that is more convenient for holding athree-dimensional sample. A knob 410 turns a large pitch worm screwmechanism (not shown) that raises or lowers the mount 404 or 504. Theworm screw has square threads, and a pin engaged in the threads attachedto the sample mount moves the sample mount up and down as the threadsturn. A handle 412 is provided for holding the container. Electricalcontacts 414 provide a path for electrical signals and power to enterthe sample container. The electrical signals can be used, in someembodiments, to control and power, for example, a sample heating coil orcooler, such as a Peltier cooler. Electrical signals can also be used insome embodiments to control a motor or other device to raise and lowerthe sample or to change the position of the sample. Connections can bemade to the container from the system by spring loaded electricalcontacts that contact the connectors on the sample container when it isloaded. In some embodiments, a sample holder can include a memory andmicroprocessor for storing and executing a program to carry out aspecified function. For example, a cup may be programmed to heat or coolthe sample. By including “intelligence” in the cup, the microscope doesnot need to be reprogrammed for different types of samples; theprogramming can be stored in the sample holder. The invention is notlimited to any particular type of sample holder.

Navigation Camera

FIGS. 6A-6C shows a configuration of electron microscope system 100including an optical imaging device, such as optical camera 602, and thelower portions of electron microscope 100. Optical imaging camera 602preferably employs a charge-coupled device (CCD) and is preferablypositioned such that the sample is observed before its environment isevacuated. The sample container 112 moves under the optical camera,which forms and stores a magnified digital image of the sample. Themagnification of the image in optical camera 602 is typically from 10×up to 100×. The optical camera 602 can move up and down to focus on thesample. Movement is preferably by an electric motor, or could bemanually. The camera has a field of view of about 8 mm by 8 mm. Multipleimages from the camera can be tiled or stitched together to create animage of a larger portion of the sample. The tiling or stitching processcan be performed automatically, with the sample being automaticallymoved under the camera by way of an electric motor into differentpositions in a serpentine pattern, each position covering a portion ofthe sample. In each position the image is captured, and all of theimages are combined to produce an optical overview of the entire sample.Alternatively, a user can control the movement of the sample to produceonly useful images. In manual mode, the user can move the sample underthe camera by touching navigation arrows on the touch screen or bypressing a point on the image can re-center the image so that the pointtouched is in the center of the field of view. Adjacent images displayedon the monitor can optionally be adjusted to provide continuity fromimage to image caused by distortion or positioning inaccuracies.

In some embodiments, the system can automatically determine the heightof the sample within the sample holder 112 based upon the focus of theoptical camera and then adjust the focus of the electron beamaccordingly. The optical camera 602 has a known focal length, so whenthe sample is in focus, the distance between the sample and the cameracan be determined. This distance is used to determine the sample heightfor adjusting the SEM focus. The height setting of the sample holder 112may be automatically communicated to a system controller, which canautomatically adjust SEM. The SEM is then automatically adjusted over acontinuous range for any working distance or magnification. Settings forthe actual sample position are determined by interpolating betweensettings for a high sample position and a low sample position. The focuscan be “fine tuned,” either automatically or manually, after beingroughly set based on the sample mount height within the sample holder.Another embodiment uses two preset height adjustments in the sampleholder; one for a wide field of view and one for a narrow field of view.Adjustment for the working distance of the objective lens is madedepending on which of the two pre-set working distances are chosen.

Sliding Vacuum Seal

Electron microscope system 100 uses a sliding vacuum seal to move thesample container into contact with vacuum buffers that remove air fromthe sample container on its way to a position under objective lens 242.

FIGS. 6A-6C shows aspects of the sliding vacuum seal. An operator loadsa sample into a sample container 112 outside of the SEM assembly 102,and sample container 112 is then inserted into electron microscopesystem 100. In one embodiment, a user slides a cover 110 to expose areceptacle for receiving sample container 112. Closing the cover 110automatically moves before sample container 112 into system 100 andunder an optical imaging system, such as optical camera 602, to acquirea low magnification image. Upon closing the cover 110, sample container112 is moved automatically into a position relative against a rigidsliding plate 605 and sample container 112 makes an airtight seal withrigid sliding plate 605, which in turn makes a sliding vacuum seal withother portions of SEM assembly 102 as described in more detail below.

As shown in FIG. 6A, sample holder 42 is held in a position against athrough-hole 46 in a sliding plate 605 to make an airtight seal betweenthe sample holder 42 and sliding plate 605. Sliding plate 605 slidesrelative to a glacier layer 610 on the base of SEM assembly 102 to movethe sample container 112 from optical camera 602 to the sample imagingposition of SEM assembly 102. A flexible plate 603 aids in making aconsistent vacuum seal with relatively uniform sliding friction. Thesliding vacuum seal is described in more detail below with respect toFIG. 7. Sliding plate 605 also moves to adjust the sample positionduring viewing under the optical camera and the electron microscope.FIG. 7, which is explained in more detail below, shows the configurationof the sample container 112 in the rigid sliding plate 605.

After a low magnification optical image of the sample is obtained fromoptical camera 602, sample container 112 moves toward the electronimaging position under objective lens 242. FIG. 6B shows that samplecontainer 112 is partly evacuated by passing under pre-evacuated vacuumbuffers 604 and 606, which remove much of the air in the samplecontainer 112 as it moves past the buffers. While two vacuum buffers areshown, different embodiments may have more or fewer vacuum buffers.

Vacuum buffers 604 and 606 provide a sufficient vacuum in samplecontainer 112 so that it is possible to acquire an electron beam imagein very little time, preferably less than two minutes, less than oneminute, less than 30 seconds or less than 15 seconds after the samplecontainer is positioned under the electron beam. Vacuum buffers 604 and606 comprises a volume, approximately one liter, that is connected tothe inlet of turbo pump 282 and that is typically evacuated before thesample is inserted. As the sample container passes the evacuation hole,air leaves the sample container and moves into the vacuum buffer,thereby partly evacuating the sample container. The pressure is reducedin the sample container roughly in proportion to the ratios of thevolume in the sample container and the volume in the vacuum buffer.Because the volumes of the pre-evacuation chambers are significantlygreater than the volume of the sample container, the pressure is greatlyreduced in the sample container, thereby greatly reducing the timerequired to pump the sample container down to its final pressure forforming an electron beam image.

When the sample container 112 is positioned below objection lens 242 forimaging, the walls of the removable sample container form part of thevacuum chamber walls, that is, the walls of the removable samplecontainer define part of the vacuum volume below the objective lens 242.The volume between the objective lens and the base plate is very small,thereby greatly reducing the time required for extraction before imagingcan begin. In a preferred embodiment, the sample container issufficiently evacuated by the vacuum buffers to begin imagingimmediately after the sample container is positioned under the objectivelens.

FIG. 7 shows the positional relationship among the components of thesliding vacuum seal. A flexible seal 702 within a groove 704 aroundsample container 112 seals with the interior of cylindrical portion 710of a flanged cylinder 712 which is attached to rigid sliding plate 605and sealed by an o-ring 716 against rigid sliding plate 605. Flexiblestainless steel plate 603 is clipped to sliding plate 605 using clip 722and moves with sliding plate 605. To move sample container 112 intoposition under the under the axis 740 of electron column objective lens246, the sliding plate 605 slides along glacier layer 610 attached to abase plate 611 of the SEM assembly 102. Springs (not shown) can beinserted into blind holes 726 in the rigid sliding plate 605 to pressthe flexible sheet 603 against the glacier layer 610 to ensure a bettervacuum seal around holes in the glacier layer, such as the holes 612that connect the sample container 112 with vacuum buffers 604 and 606and the holes through which the electron beam passes (glacier platethrough-hole 601, sliding plate through-hole 609, and flexible platethrough-hole 730). A flexible seal 720 seals flexible plate 603 relativeto rigid sliding plate 605. Using a flexible plate between the rigidplate and the microscope base reduces the sliding frictional forceneeded to slide the rigid plate and makes the sliding force moreconsistent. Using glacier layer 610 on the bottom of plate 611 reducesfriction and reduces the generation of particles.

The edges of the holes in rigid sliding plate 605 are contoured toreduce frictions, as described in a PCT/US2007/010006 to Persoon et al.,filed Apr. 27, 2007, for “Slider Bearing for use with an ApparatusComprising a Vacuum Chamber,” which is hereby incorporated by reference.The curvature is preferably such that the Herztian contact pressurebetween the moving parts will minimize particle generation. FIG. 8 showsa schematic view of the components of an electron microscope having asliding vacuum seal for moving sample holder 112 containing a sample801. A seal 817 seals a vacuum housing 811 to plate 611.

Vacuum System

A preferred vacuum system includes two vacuum pumps, external pre-vacuumpump 110 and an integral high vacuum pump such as a turbomolecular pump282 (FIG. 2). Pre-vacuum pump 110 can be, for example, a diaphragm pumpthat provides the initial pump down from atmosphere and provides anacceptable exhaust pressure for high vacuum turbomolecular pump 282. Theturbomolecular pump is preferably not contained in a separate pumphousing; instead, the turbo rotor is integrated into the electron sourceassembly 202. Different pressures are maintained in three differentzones of electron microscope system 100 using the same pumps, with thelowest pressure being maintained at the electron gun, a somewhat higherpressure being maintained between the electron source 202 and theobjective lens 242, and a higher pressure yet being maintained at thesample.

A higher pressure is maintained around the sample to prevent charging ofthe sample. Gas around the sample is ionized by the primary electronbeam and by secondary electrons, and the charged particles created bythe ionization neutralize charge that accumulated on the sample. Thepressure around the sample is preferably sufficient to neutralizecharge, while not being so great that the spot size of the primary beamis enlarged to an unsatisfactory extent. Another advantage of the higherair pressure around the sample is because the sliding seal that permitsthe sample container to be slid under the electron beam does not providean absolutely airtight seal, and as the sample container is moved underthe beam to view different parts of the sample, different amounts of airwill leak into the sample cup. By maintaining the pressure around thesample at a relatively high value, fluctuations in the air pressurecaused by movement of the sample cup 402 have less effect on imagingbecause the pressure changes are a smaller percentage of the pressure.

Pressure at the sample is preferably maintained automatically asdescribed in U.S. Provisional Pat. App. No. 60/764,192, filed Feb. 1,2006 to Slingerland et al. for “Particle optical Apparatus with aPredetermined Final Vacuum Pressure,” which is hereby incorporated byreference. U.S. 60/764,192 teaches that a predetermined vacuum pressurecan be maintained in a vacuum chamber by connecting the chamber througha known vacuum conductance to a volume at a first known pressure andthrough a second connection of a known vacuum conductance to a vacuumpump. The pressure in the chamber is determined by the ratio of firstand second known conductances, as well as the first known pressure andthe pressure at the vacuum pump inlet. As described in U.S. 60/764,192,by adjusting the relative values of the first and second vacuumconductances, a desired pressure can be maintained in the vacuum chamberwithout requiring a vacuum gauge or a control system.

In electron microscope system 100, the sample volume connects by a firstvacuum conductance to the low pressure side of the diaphragm pump, whichhas a relatively high pressure, and by a second vacuum conductance tothe low pressure side of the turbomolecular pump, which has a relativelylow pressure.

The final pressure in the sample area will be determined by the ratio ofthe first and second conductances. The final pressure can be determinedby:P _(sam)=(C ₂ /C ₁)/P _(DP)in which P_(sam) is the pressure in the sample volume; C₂ is the vacuumconductance in liters/second from the sample volume to the inlet of theturbomolecular pump; C₁ is the conductance in liters/second from thesample volume to the inlet of the diaphragm pump; and P_(DP) is thepressure at the diaphragm pump inlet.

By controlling the pressure at the diaphragm pump inlet, the finalpressure of the sample volume can be set to a prescribed value. Byadmitting gas from the inlet of the diaphragm pump, which has a pressureP_(DP) significantly less than atmospheric pressure, larger aperturescan be used compared to those that would be required is the gas wereintroduced directly from the atmosphere. The pressure in the samplevolume is preferably at least five times greater than the pressure atthe inlet to the turbomolecular pump. The gas that leaks into the samplechamber from the second vacuum conductance is preferably about fivetimes the leak that occurs from the sliding vacuum seal described below,so that fluctuations in the pressure due to movement of the slidingvacuum seal are relatively small compared to the pressure in the samplearea.

The pressure at the inlet to the turbomolecular pump and at the electrongun is preferably about 10⁻⁷ mbar. The pressure in the sample cup ispreferably between about 0.1 mbar and 50 mbar, with about 0.2 mbar beingpreferred. The pressure in the mid-column between the anode and the polepieces of the objective lens preferably operates at pressure of about10⁻⁵ mbar. An aperture in the pole pieces of the objective lens 240functions as a pressure limiting aperture to maintain a pressuredifferential above and below the lens. The anode 220 or another similaraperture maintains a pressure differential between the electron sourceand the mid-column. A beam limiting aperture (not shown) can alsofunction as a pressure limiting aperture. Skilled persons can readilyapply known technology used in environmental scanning microscopes,together with information provide herein, to produce appropriatepressures in the different portions of the chamber.

User Interface

Main Image Screen

When the sample is imaged at the high magnification of the SEM, it canbe difficult for an untrained operator to determine the location on thesample from which the image is being obtained and to understand therelationship between the image and the rest of the sample. As shown inFIGS. 9-13, the preferred main image screen 13 of the graphical portionof the user interface includes three “image windows” that remain on thedisplay to help the user put a magnified image in context. One window,referred to as the active image window 14, shows the current image.Depending upon the current operation being performed, the current imagemay be an image taken when the sample is under the optical navigationcamera 602, an image formed by the SEM when the sample is under the SEM,or an image recalled from a storage medium, as discussed below.

Another image window, referred to as the optical overview window 15,shows an image from the optical navigational camera 602. The image istypically obtained and stored before the sample holder is evacuated andbefore the sample is moved under the SEM, although the sample could alsobe moved back from the electron beam to the navigation camera ifdesired. As described above, the image in the optical overview window 15may be formed from multiple fields of view of the optical navigationalcamera juxtaposed to form a single image, or the image can be from asingle field of view of the optical navigational camera.

The remaining image window, referred to as the electron beam overviewwindow 16, shows a relatively low magnification electron beam image. Theimage in the electron beam overview window 16 is preferably obtained atthe lowest available magnification for the particular working distance.When the sample is first imaged with the electron beam, the image in thelive window and the electron beam overview window will be the same. Whenthe magnification of the active image is increased, the originalrelatively low magnification image will remain in the electron beamoverview window to provide an additional reference for the operator. Ifthe electron beam overview window is then refreshed, the SEM system willdrop down to the lowest available magnification, re-image the sample,and then return to the original increased magnification for the activeimage.

The main viewing screen 13 may include a databar 33 at the lower portionof the main viewing window 13 that shows, for instance, the date, time,magnification and scale of the current image. A data bar 33 may also beincluded on the electron beam overview window 16. Navigation arrows 29on the four sides of the active viewing window 14 allow the user to movethe image to show different parts of the sample. A user can also touchany portion of the current image to re-center the image on the touchedposition, or by “clicking and dragging” the image, should a mouse orsimilar input device be employed, as will be understood by those ofordinary skill in the art. As described above, touch screen 106 enablesa user to merely touch the display monitor 104 to activate a desiredfunction. In this case, a user may depress the image being displayed inthe optical overview window 15 and drag it to a desired position and,finally, release to set the image in a desired location within thewindow 14. A stylus may be used to this extent, or a user may simply usea finger.

Various selectable icons 19 (FIG. 9) positioned along the edges of thewindows, such as an switch icon 23 (FIG. 10) for acquiring an imageeither from the optical navigational camera 602 or the scanning electronmicroscope, an eject icon 26 for loading and unloading the sample holderinto the loading bay of the SEM assembly 102, an overview icon 22 forobtaining an optical overview of the entire sample, and various digitalpicture icons 34 for saving an image to removable media. Many of theicons shown in FIG. 9 include text that portrays the command or featurewhich the icon selectively represents. However, as shown in FIGS. 5-9,such icons may alternatively make use of an image or graphicalrepresentation to indicate their respective functions, such as, forexample, a camera icon representing a “save to digital file” function. Aselectable icon to load/unload the sample, for example, may be providedby an icon similar to that used on CD players to load/unload the sampleholder into the area of the navigational camera or into the area of theSEM. Pressing the eject icon 26 unloads the sample holder from thesystem, and the other selectable icons operate in similar fashion.

Selectable icons 19 can be used together with a rotary input device 108or other mechanical input device. For instance, when the user pressesthe magnification icon 27, for example, rotating the control on therotary input device 108 will increase or decrease the magnification.Depressing the rotary input device 108 will change the control fromcoarse magnification to fine magnification control. When the control is“fine,” an “F” (not shown) appears on the magnification icon 27 toindicate that the fine control is operating. Toggling from coarse tofine and back can be done by depressing the rotary input device control108 or by touching the magnification icon 27 on the screen 13. Thecontrols are similar for the contrast/brightness 31, focus 28, androtation 25 buttons. Regarding the contrast/brightness icon 31, pressingthe rotary input device 108 once associates the rotation control on therotary input device 108 with brightness control, and pressing the rotaryinput device 108 a second time associates the rotation control withcontrast control. Contrast and brightness can also be controlledautomatically, if the user has set those functions for automatic controlunder the settings screen (as shown in FIG. 10) discussed in greaterdetail below.

The digital picture icons 34 function to store the image displayed inthe corresponding window. The image is typically saved to a USB memorystick that plugs into the system. In one preferred embodiment, thesystem has no user accessible memory, and all images are saved to aremovable medium. In another embodiment, the system is connected to theinternet, and images can be saved to a web address or sent via e-mail.The use of removable memory makes the system particularly useful inacademic environments, in which students can use the system, save theirimages, and take their images with them or send them over the internet.

The uses of these different image windows to allow an inexperienced userto easily operate the SEM will now be discussed with reference to FIGS.5 to 9. In a preferred embodiment, once a sample has been loaded intothe SEM, the sample is transferred automatically to the optical imagingposition (as shown in FIG. 6A). The optical navigational camera is thenactivated and, as shown in FIG. 10, an optical image of the sample (aball point pen tip in this example) is displayed in the optical overviewwindow 15. A more magnified optical image is displayed in the mainviewing window 14.

FIG. 10 shows the use of two rectangular perimeter indicators; a largeperimeter indicator 43 and a smaller perimeter indicator 44 inside thelarger indicator. Perimeter indicator 43 is shown as a solid line, whilesmaller indicator 44 is shown by a dashed line. Both indicators aresuperimposed over the optical image to indicate the area of the sampleto be imaged. The larger indicator 43 serves to indicate the approximateperimeter of the entire magnified image as seen in the main viewingwindow 14. The smaller indicator 44 indicates the area of the samplethat will be imaged by the electron beam. When using a color displaymonitor, any perimeter indicators will preferably be of a color thateasily stands out as imposed against the image. One of ordinary skill inthe art will readily recognize that the perimeter indicators also servesas a reference indicator so that a user may more easily locate whicharea of the image is in fact in the magnified main viewing window 14 byreferencing the perimeter indicator imposed over the image from theoptical overview window 15. The perimeter indicators may be take othershapes such as, for example, a cross or circle.

Once the two images have been displayed, the portion of the sample to beexamined under the SEM can be moved to the center of the optical fieldof view. This can be accomplished, for example, by touching a particularpoint in either image on the touch screen display and allowing the pointto be automatically centered or by inputting instructions to move thesample, for example by using directional arrows 29 located either on thescreen or by using a keyboard (not shown). FIG. 11 shows the userinterface screen after centering the tip of the ball point pen.

After the part of the sample to be viewed has been centered, the samplecan be imaged using the SEM. Electron imaging can be selected, forexample, by using the “switch” icon 23 as shown in FIG. 11 to switchfrom optical imaging to electron imaging. Switch button 23 can beindicated, for example, either by text or by an icon showing a large anda smaller shape, each having a cross therein may also be available. Inthis example, the smaller shape represents the optical camera 602 andthe larger shape represents the electron microscope. When the button ispressed, the sample moves between the optical camera position (as shownin FIG. 6A) and the electron microscope position (as shown in FIG. 6B).An arrow on the button can be used to indicate where the sample will gowhen the button is pressed.

In FIG. 12, the image produced by SEM 93 is shown in both the mainviewing window 14 and the electron beam overview window 16. In theexample of FIG. 12, the image is initially displayed in the lowestpossible magnification. The image shown in the main viewing window 14can then be magnified as desired, for example by selecting magnificationbutton 27 and then operating a magnification slider on the screen (notshown), turning the rotary input device 108, or directly inputting adesired magnification.

The magnified image will then be shown in main viewing window 14 asshown in FIG. 13. The image in the optical overview window 15 typicallyremains unchanged after the sample leaves the optical camera and movesunder the SEM. The image in the electron beam overview window 16 istypically at a magnification that is too large to show the entiresample. When the sample is moved under the SEM, the main viewing window14 may show a portion of the sample that is outside the image in theelectron beam overview window 16. In that case, the image in theelectron beam overview window 16 can be automatically or manuallychanged to show an overview of a portion of the sample that is asuperset of the portion shown in the main viewing window 14. Forexample, if the user presses a refresh icon 29 on the main screen 14near the electron beam overview window 16, a new low magnificationelectron beam image will be obtained that is centered on the same spotas the main viewing window 14. A new image may be acquired, for example,if the image in the main viewing window 14 has been moved so that theimage in the main viewing window 14 corresponds to a point on the sampleoutside the image in the electron beam overview window 16. The lowmagnification image may be obtained by increasing the beam deflection.The low magnification image may have, for example, a 400 μm field ofview, and if the sample is moved one millimeter, it would be necessaryto obtain another low magnification image.

As described above, a perimeter indicator, such as a colored rectangleor cross, on the optical overview window 15 shows the location of theimage in the electron beam overview window 16. Similarly, a perimeterindicator 45 on the electron beam overview window image indicates theposition and preferably the relative size of the image in the mainviewing window 14 on the electron beam overview window image. Forexample, as the magnification of the main viewing window 14 isincreased, the perimeter indicator on the electron beam overview window16 will get smaller to correspond to the smaller area that is shown inthe higher magnification image in the main viewing window 14.

With the perimeter indicators as described above, a user can readilydetermine at high magnification where on the sample he or she is viewingin the main viewing window 14, thereby providing a context to even auser that is not familiar with high magnification images.

In addition to the main image screen, the preferred embodiment of a userinterface shown in FIGS. 4-9 also includes tabs 12 enabling a user toaccess the other two screens: the archive screen 17 and the settingsscreen 18 (or to return to the main image screen from either of the twoadditional screens.

Archive Screen

As shown in FIG. 9, the archive screen 17 lets the user browse andoperate on images stored on the removable memory media. The archivescreen 17 is similar to the main screen 13 and includes an active imagewindow 136 and a thumbnail gallery 131 of saved images for comparing andmanipulation. A databar 33 may be included on the archive screen 17, insimilar fashion as used in the main screen 13. The controls, orselectable icons 19, are similar to those of a digital camera memory andinclude icons for accessing a help file 35, ejecting a sample 26,deleting an image 36, holding an image to compare a selected image withother images 37, zooming 134, and browsing 133 through the imagegallery. A scroll bar 132 is also included for scrolling through thethumbnail gallery 131. The user can perform other image manipulationthat would be available on a digital camera or photo editing software.

Settings Screen

As shown in FIG. 10, the settings screen provides certain user settablefunctions. For example, the user can select which detector configurationto be used to form the electron beam images. A “fast” scan settingallows fast image refresh times, but provides images of lowerresolution. A “quality” scan setting allows slower image refresh timesand high quality resolution. Through the settings screen, a user canselect viewing a live image in “fast” mode while saving the image in“quality” mode, and delete and format the USB memory stick, or otherremovable storage medium. Date and time can be set and labels createdfor images. A user can select which format to save the image file,whether it is a TIFF, JPEG or BMP format. The user can also set whetherto automatically adjust brightness and contrast, and how often to makethe adjustments. For example, brightness and contrast might be adjustedwhenever the image is moved, or periodically. The stigmation is adjustedonce in the user settings, and then is typically stable, not requiringadditional adjustment. At least one user profile selection can beselected to store the settings of particular users for faster processingand operation times.

To maintain a simple user interface, the beam energy and current istypically preset at the factory and not adjustable by the user. Duringassembly, standard set up functions are performed, such as mechanicallyaligning to center the Wehnelt cap and filament over the anode.

In a preferred embodiment, more advanced options for controlling andoptimizing the SEM system are available but protected by a password toprevent access by less experienced operators. For example, source tiltcan be adjusted to optimize electron beam illumination intensity; astigmation control allows adjustment of the sharpness of the electronimage contours; and stage position and rotation can be calibrated stageto insure that the part of the sample being viewed is the same in bothoptical and electron imaging mode.

While the sample holder and associated method are described with respectto a scanning electron microscope, the concept is applicable to anyvacuum tool or instrument, such as a focused ion beam system.

Although the present invention and its advantages have been described indetail, it should be understood that various changes, substitutions andalterations can be made herein without departing from the spirit andscope of the invention as defined by the appended claims. Moreover, thescope of the present application is not intended to be limited to theparticular embodiments of the process, machine, manufacture, andcomposition of matter, means, methods and steps described in thespecification. As one of ordinary skill in the art will readilyappreciate from the disclosure of the present invention, processes,machines, manufacture, compositions of matter, means, methods, or steps,presently existing or later to be developed that perform substantiallythe same function or achieve substantially the same result as thecorresponding embodiments described herein may be utilized according tothe present invention. Accordingly, the appended claims are intended toinclude within their scope such processes, machines, manufacture,compositions of matter, means, methods, or steps.

1. A scanning electron microscope, comprising: a source of primaryelectrons for an electron beam; a lens for focusing the electron beam; adetector for detecting electrons from the sample; a removable sampleholder for holding a sample for observation with the electronmicroscope, the walls of the sample holder forming part of the walls ofa vacuum region containing the sample, thereby substantially reducingthe time required to evacuate the beam path to provide a vacuum adequateto form an electron microscope image of the sample.
 2. The scanningelectron microscope of claim 1 further comprising vacuum buffer volumesthat evacuate the sample container before it is positioned under thelens.
 3. The scanning electron microscope of claim 1 in which theremovable sample holder includes electrical contacts for providing poweror data to the sample holder.
 4. The scanning electron microscope ofclaim 3 in which the removable sample holder includes a microprocessorfor controller a sample holder function.
 5. The scanning electronmicroscope of claim 3 in which the removable sample holder includes aheater or a cooler.
 6. The scanning electron microscope of claim 1 inwhich the sample is maintained at a pressure of greater than 0.1 mbar.7. The scanning electron microscope of claim 1 in which the removablesample holder includes an adjustable height mount for adjusting thevertical position of the sample within the sample holder.